In recent years, large area thin-film transistor (TFT) backplanes have found important applications in the production of pixelated devices such as imaging devices, display devices, and sensor devices. TFT array backplanes generally include an array of TFTs and associated address and data lines formed on a flexible or rigid substrate, with each TFT of the array arranged to access (i.e., independently control) an associated image capturing region, sensor region or display generating region. Each TFT and its associated image/sensing/display region is typically referred to as a pixel of the imaging/sensing/display device, and each pixel is accessed during operation using well known addressing circuitry by way of the address/data lines either to transfer data from the pixel to external processing circuitry, or to transfer display data to the pixel.
TFT backplanes are typically fabricated on rigid or flexible substrates using known fabrication techniques. In some conventional imaging and display devices, TFT backplanes include a first metal layer that is patterned to form a series of gate structures and associated address lines that are formed on the substrate, a dielectric layer formed over the gates/address lines, and a second metal layer that is patterned to from source and drain contact structures and associated data lines. A conventional semiconductor (e.g., amorphous silicon) is patterned between the source and drain contacts over the gate. Image capturing or display generating portions of the imaging/display device are then formed over the TFT backplane and connected to the source contact using known techniques.
Recently organic semiconductors (examples include pentacene, α-ω-dihexylsexithiophene, poly(3-hexylthiophene), and poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene]) have been used to form organic TFT (OTFT) arrays. Such organic semiconductors are preferred due to their compatibility with flexible substrates. Using organic semiconductors also reduces fabrication costs. One reason for the lower fabrication costs is that solution-processable organic semiconductors can be patterned using jet printing, screen printing, or micromechanical molding techniques. Jet printing is described in U.S. Pat. No. 5,972,419 “Electroluminescent Display and Method for Making the Same”, WO0146987A2 “Inkjet-Fabricated Integrated Circuits”; screen printing is described in “All-Polymer Field Effect Transistor Realized by Printing Techniques” F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava Science 265 1994 p 1684-1686, and micromolding is described in U.S. Pat. No. 6,322,736 “Method for Fabricating Molded Microstructures on Substrates” which are all hereby incorporated by reference.
High performance OTFTs typically require source and drain contacts that are made from metals that allow good injection of carriers into them. For p-type organic semiconductors, such as pentacene or poly(3-hexylthiophene), good injection occurs with contact metals that have a high work function (near 5 eV), and generally do not form thick oxide layers during fabrication. Doped semiconducting polymers, such as poly(3,4-ethylenedioxythiophene) and polyaniline, have also been used to form contacts with good carrier injection properties, but these materials generally have a low conductivity relative to metals and are inadequate for circuitry that requires high conductivity interconnects. For n-type organic semiconductors, such as those based on perylene tetracarboxylic dimide or perfluoroalkyl thiophenes, low work function electrodes such as calcium may be required, but in some cases higher work function metals have been shown to be adequate. If OTFTs are fabricated solely with high conductivity metals (e.g., aluminum and copper) that generate oxides or poorly matched work functions, the OTFTs exhibit a poor electrical contact to the organic semiconductor, a parasitic contact resistance between the source/drain contact and the organic semiconductor material, which degrades the performance of the OTFT. A precious metal, typically gold, is preferably used to produce the source and drain contacts in high performance OTFTs due to its high work function and lack of a native oxide layer. However, gold may be cost-prohibitive for manufacturing displays in which large areas must be covered with the contact metal, and the waste of metal is high. Other precious metals known to form good electrical contacts to organic semiconductors include palladium and platinum, but these precious metals are typically as expensive or more expensive than gold, and therefore present the same cost-related problems in large area displays.
To minimize the high costs OTFT manufacturing associated with the use of precious metals, it has been proposed to form the OTFT source and drain electrodes using less expensive doped semiconducting polymers, such as poly(3,4-ethylenedioxythiophene) or polyaniline, or composites of them with small particles of conductors, such as carbon nanotubes, and to form the address lines from a less expensive, high conductivity metal, such as aluminum. However, these two-part approaches are complicated by the need to deposit and pattern two separate conductive materials, which increases production complexity and costs and, hence, effectively cancels the cost benefits of avoiding the use of precious metals.
What is needed is an OTFT-based backplane circuit and fabrication method that minimizes manufacturing costs while providing efficient carrier injection into the organic semiconductor.